The present invention is drawn to an electrolytic cell for the production of aluminum by fused salt electrolysis wherein electric current leaves the long sides of the cell via cathode bars which are connected to at least four asymmetric busbars which lead to the anode beam of the next cell.
Aluminum is produced from aluminum oxide by electrolysis wherein the aluminum oxide is dissolved in a fluoride melt being made up in part of cryolite (Na.sub.3 AlF.sub.6). The aluminum deposited in the process collects under the fluoride melt on the carbon floor of the cell where the surface of the liquid aluminum forms the cathode of the cell. Anodes, which are made of amorphous carbon in conventional processes, dip into the melt from above. Oxygen forms at the anodes as a result of the electrolytic decomposition of the aluminum oxide and, when carbon anodes are used, combines with the carbon to form CO and CO.sub.2. The electrolytic process takes place at a temperature range of approximately 900.degree. to 1000.degree. C.
The well known principle of a conventional reduction cell with pre-baked anodes is illustated in FIG. 1 which shows a vertical section through a part of a cell running in the longitudinal direction. The steel tank 12, which is lined with insulation 13 made of heat resistant, thermally insulating material, and carbon 11 contains the fluoride melt 10 which is the electrolyte. The aluminum 14 deposited at the cathode lies on the carbon floor 15 of the cell. The surface 16 of the liquid aluminum serves as the cathode. Embedded in the carbon lining 11, and running across the cell, are iron cathode bars 17 which conduct the direct electrical current from the carbon lining 11 of the cell to the side of the cell. Amorphous carbon anodes 18, which conduct the direct current to the electrolyte, dip into the fluoride melt 10 from above. The anodes are connected securely to the anode beam 21 by means of conductor rods 19 and clamps 20.
The electrical current flows from the cathode bars 17 of one cell via busbars, which are not shown here, to the anode beam 21 of the next cell. From the anode beam it flows to the cathode bars 17 of the cell via the anode rods 19, the anodes 18, the electrolyte 10, the liquid aluminum 14 and the carbon lining 11. The electrolyte 10 is covered with a crust 22 of solidified melt and a layer of aluminum oxide 23 on top of the crust. In practice there are spaces 25 between the electrolyte 10 and the solidified crust 22. Also at the side walls of the carbon lining 11 a crust of solidified electrolyte forms a border 24. The border 24 delimits the horizontal dimension of the bath comprising liquid aluminum 14 and electrolyte 10.
The distance d between the bottom face 26 of the anode and the surface 16 of the aluminum, also called the interpolar spacing, can be varied by raising or lowering the anode beam 21 with the jacking facilities 27 mounted on columns 28. By setting the jacking facilities 27 into operation, all the anodes are raised or lowered simultaneously. Apart from this, the vertical position of each anode can be altered individually in a conventional manner via the clamp 20 on the anode beam 21.
The electrolytic cells are usually arranged in rows, either longitudinally or transversally. The current for electrolysis flows first of all through the cells of one row, which are connected in series, and then flows back to the transformer unit through one or more neighboring rows of cells.
This feeding back of the electric current produces a vertical magnetic scattering H.sub.z, which can be estimated by the following equation which applies in general to conductors carrying an electrical current: ##EQU1## where I is the current in Ampere, and r is the average distance in cm to the neighboring series of cells.
The magnetic fields produced by the neighboring series of cells considerably disturb the desired magnetic symmetry of a reduction cell, as they combine with the magnetic fields in certain parts of the cell and in other parts cancel out the fields to a certain extent. The magnetic field produced by superposition of the different fields produces in the metal in the cell an asymmetry which, together with the horizontal components of current in the cell, is responsible for the streaming of the metal, doming and fluctuations in the metal. As all these phenomena have negative effects on the process, it is of great importance to be able to influence the distribution of the magnetic fields with the help of theoretical considerations and practical experience.
It is known that the distribution of the field in the metal in the cell can be controlled by appropriate choice of current distribution close to and around the cell. It has therefore been possible to dimension and achieve symmetry in 210 kA cells both with respect to current density and magnetic fields. However, it is necessary to consider the field distribution, not only due to effects in the immediate vicinity, but also with respect to more distant fields from neighboring rows of cells. It is in fact difficult to compensate adequately for the more distant field effects.
The expert knows, from Erzmetall, 27/10 (1974), 464, that when cells are extremely symmetrical, asymmetry must be introduced to prevent fluctuations occuring in the aluminum on the floor of the cell. This is brought about by separating the cathode aluminum conductor bars at a certain place, without depriving the cell of electric current. The separation takes place such that equal numbers of cathode bars with respect to the transverse axis of the cell deliver the current to the sides along the length of the cell.
This known process is described in FIG. 2 in which the direct current of one cell 30 is led via cathode bars 17 and cathode busbars 31 to the anode beam of the next cell, not shown here. A busbar 31 is separated at 32 which produces an asymmetry with respect to the transverse axis 33 in the cathode connections. Because of the separation, an additional magnetic field directed upwards is produced, as a result of which the magnetically induced streaming of the liquid metal can in fact be eliminated.
The patent DE-OS No. 26 53 643 describes a compensation of magnetic fields whereby the ends of the cathode bars are connected in different numbers, at least on one side of a transversely positioned cell, to the busbar leading to the anodes of the next cell. This has, with respect to creating an additional magnetic field, the same effect as separating the busbars.
In both the above cases a disadvantage arises in that the additional field which is to be produced is reduced in the next cell in the series.
It is therefore the principal object of the present invention to develop an electrolytic cell for the production of aluminum in which the interfering magnetic field from the neighboring series of cells is reduced or eliminated without impairing the superimposed magnetic field in the next cell in the series.